The String Quartet No. 6 by Béla Bartók was written from August to November, 1939 in Budapest.

The work is in four movements:

1. Mesto - Vivace
2. Mesto - Marcia
3. Mesto - Burletta
4. Mesto - Molto tranquillo

Each movement opens with slow material marked mesto (sadly). This material is employed for only a relatively short introduction in the first movement, but is longer in the second and longer again in the third. In the fourth movement, the mesto material, with reminiscences of the first movement material, takes up the entire movement (though it can be seen from Bartók's sketches that he originally intended ending with a lively dance-like finale).

This was the last piece that Bartók wrote in his native Hungary, and it is possible that it would have been his last of all, as he found it hard to compose in the United States, where he had fled to escape World War II. However, a commission from Serge Koussevitzky led to him writing his Concerto for Orchestra, and he wrote a small number of other pieces after that, as well as making a few sketches for a seventh, never completed, string quartet.

The work is dedicated to the Kolisch Quartet , and it was they that gave its premiere in New York City on January 20, 1941. The work was first published in the same year by Hawkes .